Regular mowing reaps yield rewardsIndustry Best Practice
Dr Gordon Brown reviews a long-term Turkish field study under Mediterranean conditions on the impact of different levels of weed management on apple tree growth, fruit set, yield, and profit.
In modern urbanised society, with the increasing concerns over the environment and the impact of agriculture upon it, consumers are increasingly seeking to purchase food perceived as being produced in an environmentally-friendly fashion. This has driven demand for products produced under pesticide-free ‘sustainable’ systems such as permaculture and certified organic.
Historic concerns over health issues associated with agri-chemicals, and more recently the US court cases concerning glyphosate-exposure, have put the use of herbicides under increased consumer scrutiny. The challenge for fruit production is that the alternatives to herbicides for weed control have been either less effective, labour-intensive, or expensive to apply.
The impact of alternative methods of weed control on fruit yield and size was clearly demonstrated in the research of Simon Middleton and colleagues in Queensland under HAL project AP01006 (Table 1). In this research, with the exception of weed mat and all-season hand weeding, the herbicide treatment significantly out-yielded all organically acceptable weed management strategies. Further, the herbicide treatment also yielded the second largest fruit, indicating that the alternative treatments also had a negative impact on fruit size. This highlights the importance of excellent weed control in modern high density apple orchards for them to be economically viable.
Recent research conducted in Turkey studied the impact of different levels of mowing within the tree row on the productivity of Golden Delicious (a treatment not assessed in the Australian research above). Importantly for Australian horticulture, regular within-tree row mowing is relatively simple to adopt as it can be easily mechanised with tractor-mounted implements. Hence knowledge of the impact of this treatment on orchard productivity is useful for growers considering options for reducing their reliance on herbicides.
The experimental orchard was planted in 2005 with 2800 trees per hectare and the inter row space was slashed three times per year. Irrigation was by drip irrigation every four days to fully replace evapotranspiration. Ground-based fertiliser was applied according to soil analysis. Fruit were hand thinned after fruit drop to 4.5 fruit per cm2 branch cross sectional area.
Three levels of weed competition were established by mowing, to below 5 cm, a metre-wide strip centred on the tree row. In the weak weed competition treatment, mowing was undertaken monthly for the whole of the growing season. For the moderate weed competition treatment, the mowing occurred between April and May (twice only) during flowering and fruit set. The high weed competition treatment was left unmown. Unfortunately, in this trial, no industry standard herbicide treatment was included for comparison with standard industry practices. Soil organic matter, trunk cross-sectional area (TCSA), fruit set, yield and fruit size were measured for four years from 2010 and this was followed by an economic analysis of the data.
Results and discussion
The level of weed control was found to have no significant effect on the soil organic matter or tree cross-sectional area during the measurement period. Data for fruit set is only provided for 2014 and if this is typical the large impact of level of weed control was on the percentage of flowers that set fruit with regular mowing reaching nearly 80 per cent fruit set compared to less than 40 per cent fruit set for the unmown treatment (Figure 1). Of interest is that the treatment that was only mown twice, during flowering and a month later, had intermediate fruit set indicating the importance of weed control during this important time period.
As would be expected from the fruit set data, the cumulative yield over the four years of treatment application, was highest – close to 200 tonnes per hectare (t/ha) – for the regularly-mown treatment. The average yield of 50t/ha for each of the four years makes it potentially commercially acceptable. Despite the superior fruit set in the moderate competition treatment (mown twice), compared to the unmown treatment, the cumulative yield barely differed at just under 150t/ha for the four years.
As the regularly mown treatment had higher fruit set as well as higher cumulative yield, there is potential for this treatment to yield a higher percentage of small fruit. This did not occur. Close to 70 per cent of the fruit in the regularly-mown treatment was greater than 72 mm compared to just half the fruit in the other two treatments. This indicates that regular mowing through fruit development and maturation reduces weed competition resulting in larger fruit and hence higher yield.
The economic analysis of the trial data indicates that the gross profit of the regularly mown treatment was nearly double that of the other two treatments (Figure 2).
Take home messages
- Weed competition in an apple orchard will reduce fruit set and fruit size resulting in lowered yields and reduced orchard income.
- If herbicides are to be avoided, other effective means of orchard weed control need to be undertaken if yields and income are to be maintained.
- From Australian research, which did not include regular within-tree row mowing, the most effective alternative to herbicides was weed mats or physical removal of the weeds.
- The Turkish research, which did not include a standard industry herbicide treatment, identified the importance of regular (monthly) mowing of the within-tree row area for maintaining yield and fruit quality. Early-season only mowing of this space did improve fruit set, but failed to maintain fruit size and yield.
- As tractor-mounted equipment for mowing the within-tree row space is available this represents a potentially-viable option for herbicide-free apple production with little impact on yield, fruit size and potential profit.
- Australian research is required to verify the effect of regular within-tree row mowing on yield and profitability, compared to current industry standard herbicide treatment.
About the author: Dr Gordon Brown
03 6239 6411
Turkish research – http://www.hortorumcultus.actapol.net/pub/16_5_13.pdf